Ten children with phenylketonuria aged between 9 and 11 years were studied for a period of 1 year. Their usual phenylalanine free amino acid supplement was replaced by Maxamum XP (Scientific Hospital Supplies). There was no evidence of any change in the clinical status of these children and all claimed to prefer Maxamum XP to previous amino acid mixtures. Growth was normal and various biochemical parameters of nutritional status remained unaltered.